In telecommunications and more specifically with respect to transmitting and connecting telephone calls from one subscriber to another, there is a need of separating the transmission directions with the aid of a fork constituted, for example, by a so-called hybrid circuit. This divides the incoming two-wire and two-directional line from a subscriber's set into a four-wire transmission circuit (i.e., a pair of two one-way connections) to enable the functions of signal conversion, filtering, amplification, and so forth which functions can only take place in only one direction.
When both two-wire subscriber terminals are connected together through a four-wire exchange, it is necessary that the attenuation in the four-wire loop (i.e., the loop including the hybrid circuit, both one-way four-wire paths and the exchange connection or switching stage) exceed a certain value for the avoidance of stability problems. One possibility is to arrange attenuation in the exchange switching stage, but this results in unnecessary attenuation of the signals which are to be passed through.
To obtain a sufficiently high so-called cross fork attenuation with the object of avoiding the above-mentioned stability problems, and thus obtain sufficient signal separation in the hybrid circuit, the balance impedance must accurately match the impedance in the line currently connected to the exchange via the subscriber unit. It is therefore impossible to use the same balance impedance for all subscribers, since, relative to the hybrid circuit, there can be considerable scattering of the input impedances to the different subscriber lines.
U.S. Pat. No. 3,982,080 teaches automatically adjusting the value of the balance impedance of a hybrid circuit in response to the impedance value of a given line amongst a plurality of available lines for achieving optimum impedance match between a hybrid circuit and the associated line.